The Scotland international winger provided the cross for James Morrison's opener on 52 minutes and then seized on a Danny Drinkwater
Leicester City's 20-game unbeaten home record in the Premier League came to an end as Matt Phillips scored West Brom's winner in a 2-1 victory on Sunday.
The Scotland international winger provided the cross for James Morrison's opener on 52 minutes and then seized on a Danny Drinkwater howler to grab the decisive goal 20 minutes later.
Leicester had equalised on 55 minutes through record signing Algerian international Islam Slimani.
But Claudio Ranieri's men could have no complaints about surrendering their proud record at the King Power Stadium as they paid the price for a lacklustre display.
It meant a fifth defeat of the season for last season's champions, whose sparkling form in their maiden Champions League campaign is in stark contrast to a stuttering defence of their domestic title.
Defeat leaves them in a lowly 14th position on 12 points, just two points above the relegation places.
Albion leapfrogged Leicester in the table by virtue of the win -- only their third this term -- moving to 11th on 13 points.
Jamie Vardy was again left on the bench for a league game as his long barren run in front of goal went on despite him entering the fray at half-time.
Ron-Robert Zieler, who replaced the injured Kasper Schmeichel in goal for Leicester, made an excellent start as he looked to settle any nerves, making a smart save to deny the visitors in the third minute.
He went low to his right to get his fingertips to a goal-bound cross-shot by Phillips, who had pounced on a poor touch by Andy King.
That set the tone for the opening exchanges with Leicester poor in possession and West Brom enjoying a territorial advantage.
Salomon Rondon headed just off target for the visitors from a Phillips cross.
Morrison then failed narrowly to release Chris Brunt with a well-judged through-ball which Zieler did well to slide out a clear.
But the home side did build up some momentum as the game developed and Slimani was the focal point of their attacks.
He misdirected a header from a cross by Christian Fuchs after a slick Leicester counter-attack.
The Algerian then failed by a whisker to meet a cross by compatriot Riyad Mahrez with Gareth McAuley reading the danger well and clearing before Shinji Okazaki could seize on the loose ball.
It had been a frustrating first half for the Foxes and, despite the introduction of Vardy from the bench, it got worse.
Darren Fletcher found Phillips in space and his cross deflected off Fuchs onto Morrison?s head for the midfielder to opening the scoring.
But Leicester took just three minutes to equalise as West Brom committed defensive suicide, leaving Slimani unmarked at the far post to nod home a Mahrez cross.
West Brom had a header by Craig Dawson clawed away by Zieler and Leicester were undone by Drinkwater?s awful error.
The midfielder played a horribly under-hit backpass which Phillips raced onto before finishing cleverly past a diving Zieler.